Vanishing
To everyone, it seemed like Frank was controlling his illness. However, could this still be the reason he left the show? Many believe that his conditions and the difficulties surrounding the global pandemic caused him to leave.
Self-Isolating
Because Frank has a pre-existing medical condition, he was possibly advised to take self-isolation more seriously than others. This might have meant taking a break from filming the show. However, there could have been something else stopping him from appearing.